A base station in a wireless communication system transmits signals in which there is a large difference between the average power and the peak power. As a technology to achieve high-efficiency transmission amplifiers used for transmitters in such base stations or the like, a digital transmitter has recently been studied that transforms a transmission signal into a radio-frequency digital transmission signal and amplifies such signal. As a transmission amplifier used for the digital transmitter, switch-mode power amplifiers such as Class-D amplifiers and Class-S amplifiers have been studied (NPL 1 and 2).
The switch-mode power amplifier assumes an input signal to be a pulse-shaped signal and power-amplifies the input signal with maintenance of the pulse shape. The signal amplified by the switch-mode power amplifier has frequency components except the band of the desired radio signal removed, and then it is output from the digital transmitter.
A Doherty amplifier composed of analog amplifiers such as Class-AB amplifiers and Class-C amplifiers is well known as possible means for achieving high efficiency. The Doherty amplifier is composed of a carrier amplifier that operates from a low-power region and a peak amplifier that initiates operations in a stage where the carrier amplifier has reached a saturation region. In general, the carrier amplifier is Class-AB biased, the peak amplifier is Class-C biased, and the peak amplifier becomes off state in the absence of a signal input whose amplitude is larger than a certain level.
In order to maintain high efficiency over a wider range of power levels, a Doherty amplifier as described in PTL 1 is often used that includes a plurality of peak amplifiers whose operation starting points differ from each other. The Doherty amplifier described in PTL 1 is composed of one carrier amplifier and (N−1) peak amplifiers. FIG. 2 in PTL 1 illustrates an example where N is equal to four. In the Doherty amplifier, each of the plurality of peak amplifiers is set at a different bias so that the plurality of respective peak amplifiers may initiate operations sequentially with increasing input amplitude.
PTL 2 proposes an amplifier that includes a plurality of Class-F amplifiers, which ideally operate with a high efficiency of 100% as with Class-D amplifiers, connected through quarter-wave lines, and that synthesizes the output signals of the plurality of Class-F amplifiers (FIG. 1 in PTL 2). The amplifier is designed to improve the efficiency achieving a linear amplifier by synthesizing output signals of the plurality of Class-F amplifiers.